Planets, as well as their satellites, are subject to periodic impacts by objects of various sizes, from dust and small particles to comets and asteroids. Evidence of such impacts is readily apparent in the form of craters, and such impacts have also been observed, for example the 1994 impact of Comet Shoemaker-Levy into the planet Jupiter. The effects of such impacts can be devastating. One such impact is thought to have led to the extinction of the dinosaurs at the end of the Cretaceous period. A similar impact today could potentially end human civilization, or even lead to the extinction of the human race.
In view of the threat such impacts pose to human existence, a clear need exists for methods and systems capable of defending a planet, such as Earth, from impacts from incoming bodies. Proposed methods such as launching missiles armed with nuclear warheads, however, while potentially effective, require escape from Earth's gravity well, which constrains the effective payload and/or velocity of the missiles.